Substitute the correct hal pin for ff0 as discussed in Andy's last thread. This is what he said

PID output is partly made up of FF0 * input. So just setting FF0 of the PID for each gear might have the desired effect.

I would be tempted to scale the spindle control voltage to motor speed. Assuming a 1500 rpm motor then the parameter you want out of the component is 1500/max_speed for each gear (rather than the actual top speed) then link that to the PID FF0 pin.

If this works, it could be quite a handy template for a machine with a gearbox

I have been looking at this to simplify S commanded speed to settings needed. VFD to control min to max in each gear setting
This VFD range is 40% of max and as I see it will not affect the torque very much so this is the way to get this mill going.